1. Field of the Invention
This invention relates to valves of the type for controlling wellhead flows, and it more particularly relates to through conduit gate valves with a double expanding gate assembly and floating seals.
2. Description of the Prior Art
There have been many designs of through conduit gate valves with expanding gate assemblies to insure that the wedging action of such gate elements did not occur while the valve was being adjusted between opened and closed positions. Further, it has been essential that the wedging action did occur at the opened and closed positions for providing the fluid sealing function of these valves.
The gate assembly comprises a gate member or element and segment having smooth parallel faces for engaging metal backed sealing rings. The gate element has a vee recess engaged by a complementary vee face on the segment. Axial movement between the gate element and segment causes the expansion for engaging the seals at the closed and opened positions. The gate element and segment are brought into their collapsed position while the gate assembly is moving between opened and closed positions. If the gate assembly does not remain in its collapsed position and it is moved relative to the seals, the resilient sealing rings are quickly destroyed, and also, serious injury can occur to other parts of the valve. These problems can be termed binding or back wedging.
In order to insure bringing the gate assembly into collapsed condition during movement relative to the seals, various spring biasing arrangements have been proposed. There are sophisticated theories as to the angular inclination of the vee faces in the gate assembly and spring configurations and bending characteristics to promote reaching the collapsed condition during movement of the gate assembly.
Unfortunately, these high technology arrangements of spring biasing fail when the seals "float" inwardly against the gate assembly and cause binding problems. In some gate valves, the seals are mounted in recesses with piston like clearances, and carry resilient sealing rings engaging the valve body. The fluid pressure in the conduit can force these seals inwardly against the gate. Debris, corrosion or other causes, can hold the seals in this inward position. Naturally, moving a wedging type gate against the "binding" of these seals causes sufficient drag that the conventional gate assembly cannot be returned into the collapsed condition during movement between opened and closed positions.
In other gate valves, the seals are pressed firmly into the recesses in the body. Generally, these seals are not rigidly secured in the recesses, but suffer some movement or rocking from reaction to gate assembly movements. If the seals move or float inwardly a sufficient distance to engage or wedge against the gate assembly, the resultant drag or frictional contact will cause the gate assembly to undergo reverse directional wedging action from movement of the gate assembly between its closed and opened position.
In either of the above undesired situations, dragging of the seals against the wedging gate assembly, if unable to push the seals outwardly, can produce excessive stem torque and resultant parts wear even to lock the valve in some intermediate position (not opened or closed). Thus, the valve becomes useless for its intended purposes.